I’m pretty sure the cryptographic parameters to generate a public key are included in the private key file. So while you can generate the other file from that file, it’s not only the private part in it but also some extra information and you can’t really change the characters in the private key part. Also not an expert here. I’m fairly certain that it can’t happen the other way round, or you could impersonate someone and do all kinds of MITM attacks… In this case I’ve tried it this way, changed characters and openssh-keygen complains and can’t generate anything anymore.